Device-to-device (D2D) communication is a new topic in 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) Release 12 and the main target for such a topic is to study how to realize direct D2D communication. D2D communication could happen with wireless network coverage (e.g. for commercial case) or without network coverage (e.g. for public safety). FIG. 1 illustrates exemplary D2D communications with and without wireless network coverage. On the left side of FIG. 1, UE 101 and UE 102 are within the wireless network coverage of eNB (eNode B) 103, but they are communicating with each other directly (i.e. not through eNB 103). On the right side of FIG. 1, UE 104 and UE 105 are not within any wireless network coverage, and they are communicating with each other directly.
Currently, D2D communication is not clear and needs to be studied in many aspects. In particular, at least two aspects need to be further studied. The first aspect is how to realize transition between groupcast communication and unicast communication. The second problem is how to do resource scheduling in D2D, especially in groupcast communication.